Protection of susceptible potato cultivars against late blight in mixtures increases with decreasing disease pressure

ABSTRACT Cultivar mixtures can reduce potato late blight severity on susceptible cultivars. While alternating rows of susceptible and resistant cultivars would be more acceptable than random mixtures for commercial use, they increase the genotype unit area, which is an unfavorable factor for mixture efficiency, and have been minimally efficient when disease pressure is high. The effects of disease pressure on the performance of alternating rows of cultivars possessing various types and levels of resistance were investigated in 2000 and 2001 near Quito, Ecuador, where natural pressure of late blight is high. The experiments included the highly susceptible cvs. Cecilia in 2000 and LBr37 in 2001, as well as C114 (moderately resistant) and PAN (highly resistant), planted as pure stands and as the three possible two-way combinations. Different disease pressures were obtained with three spraying schedules of a contact fungicide: nontreated, one spray every second week, and one spray weekly. The area under the disease progress curve (AUDPC) on the susceptible cultivar was 0 to 20% less in mixed than in pure plots when no fungicide was applied, 13 to 26% less with a biweekly application of fungicide, and 32 to 53% less with a weekly application. These values are comparable to those obtained in previous experiments in smaller plots with designs maximizing the distance between susceptible plants. No significant differences in mixture performance were observed according to the resistant cultivar included. Effects on yield were minimal, because of the impact of factors other than late blight. Disease pressure therefore appears as a major factor conditioning the efficiency of potato cultivar mixtures against late blight.     

关于青海省马铃薯晚疫病的发生和予测

马铃薯晚疫病(Phytophthora infestans)在青海的流行范围,基本限于东部农业区,海南藏族自治州虽能发病但不流行,柴达木地区则至今连典型中心病株亦未发现。这与各该地区的气候条件有极密切关系:海南州(如恰卜恰)7、8两月,旬平均气温皆在10℃以上,但降水仅约140毫米旬     

Occurrence of the R1 allele conferring resistance to late blight in potato R-gene differentials and commercial cultivars

A total of 23 Scottish and 14 Dutch potato R-gene differentials as well as five Austrian, two Dutch and two German commercial potato cultivars were screened for the R1 allele conferring resistance to Phytophthora infestans carrying Avr1 , via PCR amplification and sequencing. A single 1400 bp fragment with complete sequence identity to the corresponding part of the R1 allele, was obtained from genomic DNA of all potato R-gene differential clones whose denomination indicates R1 or a combination of R1 and other major resistance factors. The R1 allele was detected, as expected, in all these clones. This fragment also occurred in one Austrian and one German cultivar. Unexpectedly, the same R1 allele also was detected within all R5, R6 and R9 differentials.     

氟铃脲对番茄晚疫病的防治作用

通过药效试验发现氟铃脲对番茄晚疫病有一定的防治效果。室内离体试验结果表明,氟铃脲对番茄晚疫病菌的EC50为65.69 μg/mL;盆栽试验结果表明,6.25～200 μg/mL氟铃脲对番茄晚疫病均表现出一定的预防效果;田间小区试验200 μg/mL氟铃脲对番茄晚疫病的防效达到了64.42%。     

防治马铃薯晚疫病药剂筛选试验

选用3种药剂配方对马铃薯晚疫病进行防治试验,结果表明:施药3次,每次间隔7d,以喷施90%硫酸铜100倍液,72.2%霜霉威AS 800倍,72%霜脲.锰锌WP 600倍液轮换使用效果最佳,防效达89.0%,增产率达50.5%。     

Development of natural late blight epidemics in pure and mixed plots of potato cultivars with different levels of partial resistance

Late blight, caused by Phytophthora infestans , is the most severe disease of potato worldwide. Controlling late blight epidemics is difficult, and resistance of host cultivars is either not effective enough, or too easily overcome by the pathogen to be used alone. In field trials conducted for 3 years under natural epidemics, late blight severity was significantly lower in a susceptible cultivar growing in rows alternating with partially resistant cultivars (mixtures) than in unmixed plots of the susceptible cultivar alone. Partially resistant cultivars behaved similarly in unmixed and mixed plots. Mixtures of cultivars reduced disease progress rates and sometimes delayed disease onset over unmixed plots, but did so significantly only for the slowest epidemic. This suggests that reduction of area under the disease progress curve (AUDPC) in mixtures resulted from the cumulative action of minor effects. Disease distribution was focal in all plots at all dates, as shown by Morisita's index values significantly exceeding 1. Significant yield increases for the susceptible cultivar, and occasionally for the partially resistant ones, were observed in mixed-cultivar plots compared with single-cultivar plots. These results show that cultivar mixtures can significantly reduce natural, polycyclic epidemics in broadleaved plants attacked by pathogens causing rapidly expanding lesions.     

Inheritance of resistance to bacterial blight in 21 cultivars of rice

ABSTRACT Genetic analysis for resistance to bacterial blight (Xanthomonas oryzae pv. oryzae) of 21 rice (Oryza sativa L.) cultivars was carried out. These cultivars were divided into two groups based on their reactions to Philippine races of bacterial blight. Cultivars of group 1 were resistant to race 1 and those of group 2 were susceptible to race 1 but resistant to race 2. All the cultivars were crossed with TN1, which is susceptible to all the Philippine races of X. oryzae pv. oryzae. F(1) and F(2) populations of hybrids of group 1 cultivars were evaluated using race 1 and F(1) and F(2) populations of hybrids of group 2 cultivars were evaluated using race 2. All the cultivars showed monogenic inheritance of resistance. Allelic relationships of the genes were investigated by crossing these cultivars with different testers having single genes for resistance. Three cultivars have Xa4, another three have xa5, one has xa8, two have Xa3, eight have Xa10, and one has Xa4 as well as Xa10. Three cultivars have new, as yet undescribed, genes. Nep Bha Bong To has a new recessive gene for moderate resistance to races 1, 2, and 3 and resistance to race 5. This gene is designated xa26(t). Arai Raj has a dominant gene for resistance to race 2 which segregates independently of Xa10. This gene is designated as Xa27(t). Lota Sail has a recessive gene for resistance to race 2 which segregates independently of Xa10. This gene is designated as xa28(t).     

29个马铃薯品种(品系)抗晚疫病基因组成与抗病相关性分析

由致病疫霉引起的马铃薯晚疫病是马铃薯生产中最具危害性的病害,明确现有马铃薯品种(品系)中抗病基因组成情况对于指导抗病品种合理布局及抗病育种具有重要意义。本研究利用农杆菌介导的无毒基因瞬时表达技术分析了致病疫霉8个无毒基因在29个马铃薯品种(品系)中的分布情况。结果表明,品种间抗病基因组成各异,平均每个品种含有4个抗病基因。同时,不同抗病基因在品种中的分布频率也不同,抗病基因R1(76%)、Rpi-blb1(66%)和Rpi-blb2(66%)在品种中分布较普遍;而含抗病基因R3b(45%)、R4(38%)和Rpi-vnt1(34%)的品种较少。此外,品种中所含抗性基因的数量和品种室内抗性程度呈显著正相关(r=0.915 6,P=0.029 1)。     

Novel metrics to classify fire blight resistance of 94 apple cultivars

Fire blight (Erwinia amylovora), a potentially devastating disease in apple, can cause floral, fruit and structural damage and even tree death. Most commercial apple cultivars are susceptible and the resistance/susceptibility of many modern cultivars has not been evaluated. Fire blight resistance/susceptibility is difficult to phenotype due to quantitative resistance, impacts of tree vigour and environment on susceptibility, and the erratic nature of the disease. Resistance/susceptibility levels were determined for 94 apple cultivars and important breeding parents. In 2016 and 2017, multiple actively growing shoots per tree (about three trees per cultivar) were challenged with E. amylovora Ea153n via a cut-leaf inoculation method. Proportion of current season's shoot length blighted (SLB) was calculated for each shoot. To classify cultivar responses, estimated marginal SLB means were compared to four controls, representing highly susceptible (HS) to highly resistant (HR), via Dunnett's tests. Cultivar responses ranged from HS to HR with estimated marginal SLB means of 0.001–0.995 in 2016 and 0.000–0.885 in 2017. Most cultivars demonstrated similar resistance/susceptibility levels in both years (ρ = 0.657, P < 0.0001). K-means clustering was used to classify cultivars into three resistance/susceptibility groups based on incidence, average severity (SLB), and maximum severity values (maximum SLB and age of wood infected). Sixteen cultivars were consistently moderately resistant (MR) to HR while the remainder ranged from HS to MR. An updated comparison of susceptibility of important cultivars is provided. Resistance/susceptibility information gained could be used to identify genetic loci associated with resistance/susceptibility and/or inform parental selection in apple scion breeding programmes.     

几种杀菌剂对冬种马铃薯晚疫病的防治效果

试验结果表明:每亩用72%霜脲氰·锰锌SG100g+68.75%氟吡菌胺·霜霉威盐酸盐SC75ml、72%霜脲氰·锰锌SG100g、68.75%氟吡菌胺·霜霉威盐酸盐SC75ml和58%甲霜灵锰锌WP100g对冬种马铃薯晚疫病均有较好的防治效果,其中以每亩用72%霜脲氰·锰锌SG100g+68.75%氟吡菌胺·霜霉威盐...     

Development of potato late blight epidemics: disease foci, disease gradients, and infection sources

ABSTRACT Natural potato late blight epidemics were studied to assess the relative impact of various inoculum sources of Phytophthora infestans in Southern Flevoland (the Netherlands) from 1994 through 1996. Disease surveys were combined with characterization of isolates for mating type and DNA fingerprint pattern using probe RG57. Seventy-four percent of the commercial potato fields with early foci were clearly associated with nearby infested refuse piles. Characterization of isolates from refuse piles and fields confirmed the association. Infected seed tubers, volunteer plants, and infested allotment gardens appeared to be of minor importance for late blight development in potato fields. Several foci in refuse piles, potato fields, and allotment gardens contained more than one genotype. Due to favorable weather in August 1994, infested organic potato fields became major inoculum sources, resulting in the spread of P. infestans to adjacent conventional potato fields. Analyses of disease gradients, both at the field and regional levels, confirmed the role of the organic fields as mid-season infection sources. The mean slope of field gradients downwind of refuse piles (point sources) was significantly steeper (100-fold difference) than the mean slope of field gradients downwind of organic fields (area sources). The genotypic composition of the P. infestans populations along the gradient and of the source populations in the organic potato crops did not differ significantly. Analysis of the region gradient revealed genotype-specific disease gradients. Control measures are recommended.     

Integrated control of potato late blight: predicting the combined efficacy of host resistance and fungicides

Integrating cultivars that are partially resistant with reduced fungicide doses offers growers an opportunity to decrease fungicide input but still maintain disease control. To use integrated control strategies in practice requires a method to determine the combined effectiveness of particular cultivar and fungicide dose combinations. Simple models, such as additive dose models (ADM) and multiplicative survival models (MSM), have been used previously to determine the joint action of two or more pesticides. This study tests whether a model based on multiplicative survival principles can predict the joint action of fungicide doses combined with cultivars of differing partial host resistance. Data from eight field experiments on potato late blight (Phytophthora infestans) were used to test the model; the severity of foliar blight was assessed and scores used to calculate the area under the disease progress curve (AUDPC). A subset of data, derived from the most susceptible cultivar, King Edward, was used to produce dose–response curves from which parameter values were estimated, quantifying fungicide efficacy. These values, along with the untreated values for the more resistant cultivars, Cara and Sarpo Mira, were used to predict the combined efficacy of the remaining cultivar by fungicide dose combinations. Predicted efficacy was compared against observations from an independent subset of treatments from the field experiments. The analysis demonstrated that multiplicative survival principles can be applied to describe the joint efficacy of host resistance and fungicide dose combinations.     

Analysis of proteins differentially accumulated during potato late blight resistance mediated by the RB resistance gene

The RB gene, cloned from the wild diploid potato species Solanum bulbocastanum, confers resistance against the late blight pathogen, Phytophthora infestans. We examined changes in the proteome of potato leaves in response to inoculation with P. infestans. A nearly isogenic system comprised of susceptible Solanum tuberosum cultivar ‘Katahdin’ and resistant transgenic potato (cv. ‘Katahdin’) carrying a single copy of RB was utilized for this study. Comparative proteomic analysis revealed the presence of only 12 protein spots with a significant difference (≥ twofold) in relative abundance in resistant and susceptible potato plants after inoculation with P. infestans. Five out of the 12 identified proteins have putative roles in photosynthesis and stress responses. Silencing of these genes in Nicotiana benthamiana had no impact on RB-mediated induction of the hypersensitive response (HR). However, we found that silencing of molecular chaperone HSP90 led to the inability of RB to induce the HR after recognition of the P. infestans effector IpiO in planta.     

Exploiting host resistance to reduce the use of fungicides to control potato late blight

Field trials in 1996, 1997 and 1998 with six potato cultivars differing in levels of foliar and tuber race-nonspecific resistance to late blight were treated with 100, 50 and 33% of the recommended dose of the fungicide fluazinam at application intervals of 7, 14 and 21 days. Using a mixed inoculum of six or seven indigenous isolates of Phytophthora infestans small potato plots were inoculated via infector plants. A foliar blight model for the relationship between the effects of resistance, fungicide application and disease pressure was developed using multiple regression analysis. Cultivars with a high level of quantitative resistance offered the greatest potential for fungicide reduction. The model showed that the effect of resistance on integrated control increased exponentially with increasing cultivar resistance. Reducing fungicide input by lowering the dose resulted in less foliar disease than extending application intervals. The higher the disease pressure, the greater the risk associated with reducing fungicide input by extension of application intervals. The field resistance of cultivars to tuber blight mainly determined the frequency of tuber infection. Exploiting high foliar resistance to reduce fungicide input carried a high risk when cultivar resistance to tuber blight was low. When field resistance to tuber blight was high, a medium level of resistance in the foliage could be exploited to reduce fungicide dose to c . 50%, provided application was at the right time. At a high level of field resistance to both foliar and tuber blight, application intervals could be extended.     

Chitinase levels in rice cultivars correlate with resistance to the sheath blight pathogen Rhizoctonia solani

Various rice cultivars were selected and screened for their reaction to sheath blight in the greenhouse. Cluster analysis of percent relative lesion height (% RLH) generated four groups of cultivars with a coefficient of similarity of 3.27. Chitinase activities were detected 24 h after inoculation of moderately resistant cvs Betichikon, Dudruchi, Khatochalani, Padi Pulut Malat, Kakua, IR72, Khakibinni. But in the susceptible cv. IR58, chitinase activity was detected only 36 h after inoculation. Western blot analysis showed that class 1 and class 2 chitinases were induced following Rhizoctonia solani infection of these cultivars. The % RLH and the number of infection cushions were negatively correlated with the level of chitinase activity. Moderately resistant rice cultivars had higher levels of chitinase activity and lower disease severity and numbers of infection cushions formed compared to IR58.     

Effects of temperature and light on non-race-specific resistance of potato leaflets to late blight

Effects of temperature and illumination on colonization by Phytophthora infestans of detached leaflets of five potato cultivars differing in field resistance to blight were investigated using an ELISA system to quantify the pathogen. Leaflets of cvs Teena and Shelagh, and in one experiment cv. Brodick, were more resistant to colonization when infected leaflets were incubated at 10°C than at 20°C, but temperature conditions before inoculation had little effect. Both photoperiod and light intensity during illumination of intact plants before inoculation interacted with genotype to determine subsequent colonization of infected leaflets incubated in darkness. Leaflets from plants of cv. Teena were more resistant to colonization after exposure to low, rather than to high, light intensities but photoperiod had no apparent effect. Leaflets of cv. Shelagh grown in a 20 h day were more resistant than those grown in a 10 h day but light intensity had no effect. Leaflets of cv. Brodick were more resistant after a 20 h day than a 10 h day and after exposure to low, rather than to high, light intensities. Leaflets of cv. Bintje were extensively colonized and those of cv. Torridon remained relatively resistant to colonization, irrespective of temperature and lighting conditions. There was a high level of unexplained variation in all the experiments.